Rotary regenerative air preheaters are typically used to transfer heat from a flue gas stream exiting a furnace, to an incoming combustion air stream to improve the efficiency of the furnace. Conventional preheaters include a heat transfer sheet assembly that includes a plurality of heat transfer sheets stacked upon one another in a basket. The heat transfer sheets absorb heat from the flue gas stream and transfer this heat to the combustion air stream. The preheater further includes a rotor having radial partitions or diaphragms defining compartments which house a respective heat transfer sheet assembly. The preheater includes sector plates that extend across upper and lower faces of the preheater to divide the preheater into one or more gas and air sectors. The hot flue gas stream and combustion air stream are simultaneously directed through respective sectors. The rotor rotates the flue gas and combustion air sectors in and out of the flue gas stream and combustion air stream to heat and then to cool the heat transfer sheets thereby heating the combustion air stream and cooling the flue gas stream.
Conventional heat transfer sheets for such preheaters are typically made by form-pressing or roll-pressing a sheet of a steel material. Typical heat transfer sheets include sheet spacing features formed therein to position adjacent sheets apart from one another and to provide structural integrity of the assembly of the plurality of heat transfer sheets in the basket. Adjacent pairs of sheet spacing features form channels for the flue gas or combustion air to flow through. Some heat transfer sheets include undulation patterns between the sheet spacing features to impede flow in a portion of the channel and thereby causing turbulent flow which increases heat transfer efficiency. However, typical sheet spacing features are of a configuration that allows the flue gas or combustion air to flow through open sided sub-channels formed by the sheet spacing features, uninterrupted at high velocities and with little or no turbulence. As a consequence of the uninterrupted high velocity flow, heat transfer from the flue gas or combustion air to the sheet spacing features is minimal. It is generally known that causing turbulent flow through the plurality of heat transfer sheets such as through the channels defined by and between adjacent sheet spacing features increases pressure drop across the preheater. In addition, it has been found that abrupt changes in direction of flow caused by abrupt contour changes in the heat transfer sheets increases pressure drop and creates flow stagnation areas or zones that tend to cause an accumulation of particles (e.g., ash) in the flow stagnation areas. This further increases pressure drop across the preheater. Such increased pressure drop reduces overall efficiency of the preheater due to increased fan power required to force the combustion air through the preheater. The efficiency of the preheater also reduces with increasing weight of the assembly of heat transfer sheets in the baskets due to the increased power required to rotate the flue gas and combustion air sectors in and out of the flue gas and combustion air streams.
Accordingly, there exists a need for improved light weight heat transfer sheets having increased heat transfer efficiency with low pressure drop characteristics.